This invention relates to the art of power supplies and, more particularly, to a circuit for use in a high power half bridge converter which uses field effect transistors (FETs).
Power converters are known in the art and typically serve to accept energy from an unregulated energy source, such as a voltage source, and derive therefrom a regulated, isolated voltage which is applied to a load circuit. The regulation function is performed by interposing a regulating device between the source of energy and the load circuit.
One form of regulating device known in the prior art includes switching type regulating devices interposed between the source of energy and the load. These operate in a discontinuous manner in controlling the rate of energy transmission and, hence, consume less power during the regulating operation than regulating devices of the variable impedance type. The switching device has two modes of operation, fully on and fully off. The switching device is periodically turned on for a time interval to permit energy transfer for purposes of maintaining the power output at a predetermined level.
Typically, such switching type regulating devices employed in power converters utilize semiconductor devices, such as power transistors, as the switching devices. These devices are turned fully on, or saturated, or fully off during operation. When fully on, the semiconductor devices are conducting and little or no power is dissipated. Also, when nonconducting or fully off no power is dissipated therein. Power is, however, dissipated in such a semiconductor device during the time interval of switching from a nonconducting condition to a conducting condition and vice versa. It is during the switching time interval that a substantial amount of power may be dissipated in such a semiconductor device, and if large enough this may severely damage the semi-conductor device.
It is desirable to provide power converters of the switching type which are cost effective and which occupy a small amount of space. This, then, necessitates high frequency operation, such as in excess of 20 KH.sub.z and preferably at substantially higher frequencies, such as 250 KH.sub.z, while transferring large amounts of power, such as on the order of several thousand watts or more. In order to achieve such high frequency operation of power switches, sinusoidal operation in the form of a series resonant converter has been employed as opposed to the squarewave operation typically employed in the prior art.
One such example of a series resonant converter including a control circuit therefor is shown in U.S. Pat. No. 4,648,017 entitled "Control Of A Series Resonant Converter" which issued on Mar. 3, 1987 and is assigned to the same assignee as is the present invention (hereinafter the '017 patent). As described in the '017 patent, the converter uses a series half bridge arrangement having a single power transformer for the power switches. As described in more detail hereinafter that configuration may give rise to spurious radio frequency signals which may either interfere with the operation of the supply or radiate outside of the supply to surrounding equipment. It is desirable to minimize those signals.
As described in the '017 patent, the control circuit therein initiates the turning off of the on FET only when the resonant current becomes zero. A sample of the resonant current is provided as an input to the control circuit. That sample must contain therein the information relating to the going to zero of the resonant current. As described in more detail hereinafter, the conventional techniques for sensing resonant current do not, under certain circumstances, provide a sample which contains that needed information.